This was definitely interesting. I'll start off by saying I'd recommend the game at 30% off or better, as I got it for $4.89 and I feel that price was solid. The game made me want it to end faster, yet I wanted more by the end.
The major problem is, the game is not "there", not polished and ideal. Voice acting left a lot to be desired. Honestly, I picked it up as a fan of Harry Callahan's music, and a fan of Portal. The music is pretty great, but attaching it to in-level speakers means the volume is very inconsistent. Having your ears blown out by standing near the speakers is realistic, but the difference between max volume and minimum is too much. Maybe it's tied to Source, but I'd like to see the audio rebalanced.
Gameplay has some neat elements, but I think being gels-only makes puzzles too limited. There were several times I thought "Was that the solution?" as the technique I used didn't seem intentional. Gameplay is not the thoughtful style of Portal, but far more platforming. At the same time, gel is easy to apply everywhere, compared to limited portal placement options in the base game. This ability to coat most levels in gel enabled several boundary glitches and shortcuts. I was hoping for more interesting levels, as many were very flat in layout, where vertical challenges would've been nice. Workshop support does help to add more content, but that still doesn't add to the story. I feel like there must be more to the storyline, and I'd like for more content to be added.
A bit better story, stronger voice acting, and some level fixes would go a long way to make this game really appealing. Although the middle-section of the game made me wish it would wrap up faster, the ending left me begging for more. It's probably 4 hours long for most, putting 30% off as a good price-per-hour and enjoyment value. I'd like to see Valve show some stronger support for the mod, maybe connecting it to an ARG to show some information for Portal 3, but that's just a dream and a wish. But THAT would get me excited. For what is available here, it's decent enough as long as you accept that it won't have Valve's level of polish and quality.